The invention is in the field of galvanically isolated current measurement and concerns a fully packaged integrated current monitor system and a method for manufacturing such encapsulated micro systems.
A convenient principle of galvanically isolated current measurement is the detection of the magnetic field generated by a current. This magnetic field is e.g. measured with the aid of a Hall sensor which is e.g. integrated on a semiconductor substrate. Such systems are e.g. fully packaged in an IC-housing containing a die with integrated sensor means and a current path for the current to be monitored.
The publications DE-3828005 and WO-95/25959 describe systems according to the above principle in which the die with the integrated sensor means is mounted on a chip support. This chip support consists of an electrically conducting material and extends out of the IC-housing and is used as a path for a current to be measured. These systems are applicable for measurements in a large current range. However, the shape of the chip support (and consequently the current path) is dictated by its mechanical supporting function. Due to this constraint, the chip support cannot be optimized for its function as current path and therefore, an only limited sensitivity can be achieved. Furthermore, there is always the semiconductor substrate and an additional insulating layer between the current path and the chip with the magnetic sensor, i.e. a distance of approximately 0.7 mm between the current path and the sensor means. As the magnetic field generated by the current decreases drastically with the distance from the current path the sensitivity of the system is further limited.
The publication WO-95/25959 also suggests to integrate the current path on the structured surface of the die close to the integrated magnetic sensors. Such a system can be packaged and contacted in exactly the same way as any conventional die. However, besides increasing the size of the die and therewith the cost of the die, the measuring range of the system is greatly reduced compared with the measuring range of a system in which the chip support serves as current path.
Regarding the measuring range, the same applies to the system as described in the publication U.S. Pat. No. 5,041,780 in which the current path is realized as an additional structured metallic layer deposited on the structured surface of the die containing the integrated magnetic sensors. Such a current path is structured for optimum relative positioning of the magnetic sensors and current path and allows positioning of the sensors considerably closer to the current path (high measuring sensitivity) than other systems. However, manufacturing the die needs additional manufacturing steps (deposition and structuring of additional layers) which add to the overall costs of the die.
It shows that all the known fully packaged systems for current monitoring and the methods for manufacturing them have their drawbacks regarding either measuring range or measuring sensitivity or cost. This means that the state of the art does not allow to manufacture a low cost system having at the same time a very large measuring range and a very high measuring sensitivity.
One object of the invention is to provide a fully packaged current monitor system for galvanically isolated current measurements which system is encapsulated within an IC-housing and comprises integrated magnetic sensor means with corresponding electronics and an electrically conducting path for the current to be measured and which system avoids the above-described shortcomings of the prior art devices, i.e. covers a large current range with high sensitivity and can still be manufactured at substantially the same or even lower costs than the known devices.
Another object of the invention is to show a method for low cost manufacturing of the inventive fully packaged current monitor system for galvanically isolated current measurement.
This object is achieved by the system and the method as defined in the claims.
The inventive system covers a current range up to 50 A for the current to be measured. Furthermore, high measuring accuracy and high sensitivity is achieved due to a very small distance between magnetic sensors and the path of the current to be monitored. The inventive system can be manufactured using state-of-the-art equipment, in a minimum of manufacturing steps and with no special adaptation of the equipment. Therefore, high volume and low cost manufacturing is ensured.
The novel structure and the inventive method of manufacturing are based on the idea of modifying a standard lead frame by short cutting two or more leads to form a current path. This current path of the modified lead frame together with the leads for electrical interconnection serve as chip support on which the die is mounted. Thereby, the die is mounted with the integrated magnetic sensors and the bonding pads towards the current path and the leads whereby lead-on-chip packaging technology (LOC) is used. The necessary glue tape for the LOC packaging technology additionally serves as insulating layer between current path and sensor die to ensure a high breakdown voltage required for galvanically isolated current measurements. This packaging sequence is preferable carried out on a commercial LOC die bonder.
The inventive current monitor system shows very low power dissipation in the path for the current to be measured. Therefore, the system is applicable for currents up to 50 A without exceeding the operating temperature of the integrated magnetic sensors. Due to the perfect insulation between the current path and the die with the integrated magnetic sensors a breakdown voltage in the order of several kV is achieved. The system response in a current range of xc2x110 A exhibits a small non-linearity below xc2x10.3%. This results in an absolute measurement accuracy better than 50 mA (sensitivity in the order of 0.1 mV/A, resolution in the order of 10 mA). The performance can be further improved by soft ferromagnetic field concentrators.
Above all, the inventive manufacturing method of the current monitor system can be realized with well known and established equipment (in particular a LOC die bonder, a wire bonder, and equipment for plastic molding). The manufacturing flow requires neither an additional process step nor an adaptation of the equipment compared with the manufacturing flow of packaging a conventional die.
Generally spoken, the inventive method comprises the steps of:
providing a die with integrated magnetic sensors, optional integrated electronics, and bonding pads for external electrical connection,
providing a lead frame comprising a frame, leads for electrical connection to the bonding pads attached to the frame, and a current path formed by short cutting at least two leads,
mounting the lead frame onto the structured side of the die by connecting the free ends of the leads and the current path to the structured side of the die with the aid of electrically insulating connecting means placed between areas of the lead frame and the die to be connected,
connecting by wire bonding the bonding pads of the die to the leads of the lead frame indented for electrical interconnection,
encapsulating the die and the parts of the lead frame connected to the die in an IC-housing,
and trimming off the frame of the lead frame.
The electrically insulating connecting means advantageously consist of a glue tape as used in lead-on-chip or LOC packaging technology. Such glue tape usually consists of three polymer layers, whereby the outer layers have a lower adhesive glass transition temperature than the center layer. For the step of connecting, the tape is brought to a temperature above the lower adhesive glass transition temperature of the outer layers and below the adhesive glass transition temperature of the center layer and the parts to be connected are pressed together. Using such connecting means ensures an insulating layer having a defined thickness between parts of the lead frame and the die. Glue tapes as used in LOC packaging technology give an insulating layer having a typical thickness of e.g. 0.05 mm to 0.1 mm.
For the step of mounting the lead frame on the die, a known and well established LOC die bonder can be used without any special materials or settings. The LOC packaging technique is widely used for assembling memory cells to increase the ratio of die size to overall packaging dimensions. Thanks to this application LOC technology has become a way of inexpensive and reliable high volume production packaging, which according to the invention is newly open for producing current monitor systems.
Advantageously, the magnetic sensors are Hall sensors sensitive perpendicular to the die surface and for maximum sensitivity they are positioned in locations of maximum magnetic amplitude perpendicular to the die surface, i.e. with the center of the sensitive plane of the sensors near the longitudinal edge of the current path.